A non-contact type IC card is capable of easily exchanging information with a device which performs reading and writing of the information (referred to as a non-contact type IC card reader or a non-contact type IC card reader/writer) by wireless communication. The non-contact type IC card is provided with many superior features such as a larger memory capacity in comparison with a magnetic card and a high tolerance against fraudulent reading and tampering of stored information. Therefore, a non-contact type IC card has been widely and generally utilized in recent years in applications such as a credit card for a financial institution, a card for storing electronic money, a commuter pass for means of transportation.
In these systems, there are many cases that no cell is provided in a non-contact type IC card and the non-contact type IC card is operated by non-contact power feeding through an electromagnetic wave emitted from a non-contact type IC card reader (see, for example, Japanese Patent Laid-Open No. 2004-356765 and Japanese Patent Laid-Open No. 2005-136944).
In the systems of the above-mentioned applications, bidirectional data transmissions are required, in other words, a direction from a non-contact type IC card reader to a non-contact type IC card and a direction from the non-contact type IC card to the non-contact type IC card reader are required. In the direction from the non-contact type IC card reader to the non-contact type IC card, data transmission is performed by carrying out ASK (Amplitude Shift Keying) modulation of a carrier emitted from the non-contact type IC card reader.
On the other hand, in a direction from the non-contact type IC card to the non-contact type IC card reader, the non-contact type IC card is not provided with a power supply (cell) and thus the non-contact type IC card is not provided with a function for emitting a carrier. Therefore, in a non-contact type IC card, data transmitting function is realized by a so-called “load modulation system” in which a carrier emitted from the non-contact type IC card reader is received by a load resistance connected with a tuning circuit structured of an inductor and a capacitor and a value of the load resistance is varied corresponding to “1”/“0” of emitted data.
The non-contact type IC card reader detects a voltage variation of its antenna coil generated depending on a variation of the value of the load resistance of the non-contact type IC card and data from the non-contact type IC card to the non-contact type IC card reader are reproduced.
As described above, the non-contact type IC card reader is required to supply electric power to a non-contact type IC card when communication is performed. An electromagnetic wave is generated as means for supplying electric power. However, this system is classified as a radio system and thus radioactive emission of an electromagnetic wave level (transmission power level) is required to keep within a specified value prescribed by Radio Law in each country.
In order to secure a communication distance with a non-contact type IC card, a sufficient electromagnetic wave is required. The higher the electromagnetic wave level (transmission power level) is, the radiation level from a product becomes larger and thus a margin becomes insufficient for the specified value of Radio Law (radioactive emission) or may exceed the specified value.